The "Allow multiple values" option only work for ',' AND operator
Here I am considering the same example from
Allow multiple value - Issue →
.
1. Drupal standard installation.
2. Add 3 terms to the tags vocabulary (e.g. Foo, Bar and Foobar).
3. Add new taxonomy reference field to the User: "Visible articles".
4. Edit current user, add just "Foo" and "Bar" to the "Visible articles".
5. Create 4 new articles, 1 tagged "Foo", 1 tagged "Bar", 1 tagged "Foobar", 1 tagged "Foo" and "Bar"
6. Create a view listing all articles.
7. Add a contextual filter for article "Tags".
8. "Provide a default value"; choose "User Field Value" (of course) and choose field "field_visible_articles".
9. In the very bottom now under "MORE" choose "Allow multiple values".
10. Save and check view.
The view shows "Foo" and "Bar" article(s).
In the Default argument plugin, multiple values are imploded with comma-sign if "Allow multiple values" is chosen.
I wanted to use the default use case where values should be imploded with the '+' sign (OR), so I have unchecked the 'Allow multiple values' checkbox. Due to that getArgument method returned a single value (first value from an array) and I have got the wrong result.
To solve this problem, I have built a patch that provides the option to handle multiple values.
Please feel free to correct my understanding!